The neural, hormonal, and metabolic mechanisms involved in the elevation of arterial blood pressure observed in the squirrel monkey during operant conditioning procedures at low environmental temperatures will be studied using naive and trained unanesthetized subjects, with and without surgical denervation of sino-aortic baroreceptors. Arterial blood pressure, heart rate, cardiac output, and the distribution of cardiac output will be studied by means of catheters chronically implanted in the abdominal aorta, right atrium, and left ventricle. Hormonal responses (renin, angiotensin, aldosterone, and catecholamines) will be measured using arterial blood samples and immunoassay techniques. Metabolic and respiratory responses will be studied by measuring body temperature, oxygen consumption, and pulmonary and systemic arterial pO2, pCO2 and pH.